One problem in wireless communication is that of multipath propagation, a phenomenon in which the original transmitted signal arrives at the receiver accompanied by a number of its heavily attenuated replicas each with a different magnitude, phase and time-of-arrival. Multipath propagation occurs due to the transmitted signal traversing various echo paths between the transmitter and receiver.
Channel estimation refers to various techniques for identifying the response of the multipath channel to a unit-pulse signal in either time or frequency domain so that this response information can be used in the detection process to improve the information reliability at the receiver.
Frequency-selective multipath propagation results when different harmonics of the transmitted signal are attenuated differently due to destructive or constructive interference between the signal replicas traversing different paths. If multipath propagation is frequency-selective then it is no longer possible to apply the same conventional time-domain channel estimation techniques to the entire frequency range of the received signal. This makes channel estimation much more complex, and is often referred to as the frequency selectivity problem.
Multicarrier communication is a natural and promising solution aimed at solving the frequency selectivity problem. In multicarrier communication, the transmission is divided into parallel, fixed sub-channels (also called subcarriers) with narrow enough bandwidths so as to make them almost frequency-flat, i.e., the effects of frequency-selective channel conditions can be considered as constant over a specific sub-channel. Thus, instead of sophisticated serial equalisation techniques to compensate for channel impairments at the receiver, one can employ relatively simple block-wise frequency domain equalisation techniques. To reliably recover the transmitted data, accurate estimation of the channel state information at the receiver is required to be able to determine the equaliser coefficients. Channel state information can be determined by identifying the channel frequency response or the channel impulse response.
Among the channel estimation techniques, training-based, in particular pilot-assisted methods, have gained noticeable popularity due to their simplicity, reliability and adaptability. Pilot-assisted channel estimation relies on a small number of pilot symbols being multiplexed along with the data into the transmitted signal. Channel response parameters (e.g. the required equaliser gain and phase shift for each subcarrier) can be estimated by filtering and interpolating the received pilot symbols.
Recently there has been extensive research into establishing optimal pilot pattern design for conventional multicarrier systems, such as orthogonal frequency division multiplexing (OFDM) systems operating under white Gaussian noise conditions. It has been shown that in OFDM systems, maximisation of the average lower bound on the transmission capacity, which is equivalent to the minimisation of channel estimation minimum mean square error (MMSE), is achieved only through an equally spaced (ES) pilot pattern with equal power distribution among the pilot symbols.
Cognitive radio refers to wireless communication applications that adaptively utilize unoccupied frequency bands by means of a technique known as dynamic spectrum access (DSA). Dynamic spectrum access permits secondary band (unlicensed) users to communicate without affecting primary band (licensed) user transmissions. An example of such a system is described in US published patent application number 20090274081 in the name of T. S. Kwon et al entitled, “Device to Sense Frequency Band to Share Operating Frequency Bands in Heterogeneous Communication Systems and Method thereof”.
Recently, it has been proposed to use multicarrier systems in dynamic spectrum access cognitive radio applications. Dynamic spectrum access systems introduce a great range of uncertainties with regard to the optimal pilot pattern design, due to the inherent adaptation requirements for the transmitted signal. In cognitive radio applications, secondary band users may experience interference from primary users. This interference can originate from remote high-power transmission sources or from other system spectrum side lobes in the adjacent frequencies. As a result, cognitive radio multicarrier systems are forced to operate under conditions of arbitrarily located virtual subcarriers (i.e. subcarriers unsuitable for transmission) and non-white additive noise (i.e. white Gaussian noise plus interference). This makes all the previous optimal pilot pattern designs generally inapplicable to dynamic spectrum access multicarrier systems.